The adsorption and the reaction of NH3 and NOx on supported V2O5 catalysts: effect of supporting materials

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Abstract
The effects of supporting materials (Al2O3, SiO2, TiO2) on the catalytic activity and selectivity of V2O5 in the reduction of NOx with ammonia are reported. The adsorbed state and the reactivity of the surface species were examined by an ir spectroscopy together with volumetric measurements, and the electronic state and the surface composition were studied by XPS.The reactivity of an adsorbed ammonia (NH4+) for NO2 over the supported catalysts was in the following increasing order, TiO2 > Al2O3 > SiO2. NH4+(ad) and NO2(ad) were both reactive species in the reaction over V2O5, V2O5/Al2O3, and V2O5/TiO2. Among these catalysts, the oxidized surface of V2O5/TiO2 adsorbs NO as NO2(ad), i.e. the reactive species, whereas other catalysts did not adsorb NO unless oxygen coexisted in the gas phase. In the case of V2O5/SiO2, the reaction between NH4+(ad) and NO2(ad) was very slow, while the surface oxygen was easily removed by the reduction with NH3 and resupplied by the introduction of NO on the surface. This leads to the N2O formation through so-called redox mechanism. The NH4+(ad) on V2O5/SiO2 showed an ir absorption band at 1435 cm−1 which was much higher than those on other catalysts, and XPS measurement suggested that V2O5 on SiO2 is electrically isolated.

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